But according to the new rule, Trump has authority to restrict illegal immigration "if he determines it to be in the national interest."

Trump's administration argues that he has the executive power to curb immigration in the name of national security, a power he invoked right after taking office with a controversial ban on travelers from several mostly-Muslim countries - whose final version was upheld by the US Supreme Court on June 26 after a protracted legal battle.

"Today's rule applies this important principle to aliens who violate such a suspension or restriction regarding the southern border," Secretary of Homeland Security Kirstjen Nielsen and acting attorney general Matthew Whitaker said.

Kirstjen Nielsen.

The New York Times

Those seeking political or other kinds of asylum - nearly all of them coming from impoverished and violent crime-plagued countries of Central America - will be heard exclusively at the border crossings, administration officials told journalists.

This is expected to put a dent in those streaming into an already overburdened system, officials said, noting that there is a backlog of more than 700,000 cases in the immigration courts.

Campaign controversy

Many politicians on both sides of the aisle agree that the US immigration system is hugely inefficient and unable to cope with demand. However, Trump's focus on the issue during campaigning for Tuesday's hotly contested midterm congressional elections was criticized as veering into immigrant-bashing and even racism.

In speeches and on Twitter, Trump hammered away nearly daily at "caravans" of a few thousand impoverished Central Americans that periodically attempt to walk up through Mexico and then gain entry to the United States.

He called a current caravan, which is still hundreds of miles from the US border and dwindling in numbers, an "invasion" and said it would bring hardened criminals to US streets.

Administration officials say that aside from the rhetoric the border really does have a problem, given that anyone who manages to get across can request asylum and subsequently often vanish while their case sits in the court system.

Members of the migrant caravan arrive at the shelter of the Church of the Assumption, in the city of Puebla.

AAP

"The vast majority of these applications eventually turn out to be non-meritorious," a senior administration official said, asking not to be identified.

Less than 10 per cent of cases result in asylum being granted, the government says.

Human rights campaigners and other critics of the Trump crackdown say that by restricting asylum seekers to the narrow border crossing points - which are already under enormous pressure - the government is effectively shutting the door on people who may truly be fleeing for their lives.

But the administration official argued that "what we're attempting to do is trying to funnel credible fear claims, or asylum claims, through the ports of entry where we are better resourced."

That way, he said, courts will "handle those claims in an expeditious and efficient manner, so that those who do actually require an asylum protection get those protections."

In 2018, border patrols have registered more than 400,000 illegal border crossers, homeland security said. And in the last five years, the number of those requesting asylum has increased by 2,000 percent, it said.